Active Power Filter with Soft Switching

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ELECTRONICS, VOL. 14, NO. 1, JUNE 2010
93
Active Power Filter with Soft Switching
Petr Šimonik
Abstract—The paper describes used techniques, operation
characteristics and develops information of the unconventional
soft switched active power filter. The power part is based on
progressive IGBT’s and high speed voltage and current sensors.
The APF is connected in parallel to the AC input of the system,
and corrects all loads directly from the AC main. Control unit is
based on use of digital signal processor. Used conception brings
the savings of electrical energy and better EMC properties in
comparison with common kinds of APF. The active power filter
can be used for suppression of line current harmonics at AC
main.
Index Terms—Active filter, harmonics, harmonic distortion,
resonant converter, resonant DC link, soft switching, ZVS
converter.
transformers, malfunction of automatic control system,
damages to capacitor due to resonance, interference in
telecommunication systems, voltage distortion and lagging in
power factor, and others. However, a flexible and versatile
solution to power quality problems is offered by active power
filters.
II. FILTER DESCRIPTIONS, POWER PARTS
This work describes an implementation of parallel active
power filter (PAPF) aimed at correcting current harmonics at
power supply system. PAPF compensates current harmonics
Device with
frequency
Computers
I. INTRODUCTION
T
HE usage of power electronics (switch mode power
supplies, adjustable speed drives, and so on) and also the
usage of modern and new-fashioned electronic devices has
been increasing rapidly on the all world. Modern and newfashioned devices impose nonlinear loads to the AC main that
draw reactive and harmonics current in addition to active
current. The reactive and harmonics current lead to low power
factor, low efficiency, harmful electromagnetic interference to
neighborhood appliance.
As an alternative parallel harmonics correction technique
can be use parallel active power filter with resonant DC-link
that will be describe in this paper. This parallel active power
filter is a device that is connected in parallel to compensated
devices and cancels the reactive and harmonics currents from a
group of these nonlinear loads so that the resulting total
current drawn from the AC main is sinusoidal. The validity of
the design methodology is confirmed by use of PSpice
computer simulation and by real site testing on the practical
realized model of parallel active power filter with soft
switching.
Power quality problems are common in most commercial,
industrial and utility networks. Resulting problems from
current harmonics can by varied, but typically relate to
performance, safety and overheating. Power supply system
polluted by current harmonics can lead to – overvoltage/current in the power supply system, over-heating in
distribution system due to skin effect, iron losses in
P. Šimonik is with VŠB – Technical University of Ostrava, Faculty of
Electrical Engineering and Computer Science, Ostrava, Czech Republic
(e-mail: [email protected]).
Electric lights
3. Phase rectifiers
Load Current
Without
Paralel
Line Current
Fig. 1. Block diagram of devices connection.
Load Current
Parallel
APF
Line Current
With
Parallel
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ELECTRONICS, VOL. 14, NO. 1, JUNE 2010
by injecting equal, but opposite harmonic compensating
current. This principle is applicable to any type of load
considered a harmonics source.
PAPF is normally implemented with PWM voltage source
inverters. In this case the PAPF using converter with resonant
DC-link operates as a voltage pulse source, so current is
enforced by resonant voltage pulses. Typical controls of PAPF
are linear current control, digital deadbeat control, and
hysteresis control. However, most methods for obtain right
control require high-speed digital signal processor system with
fast A/D. In this type of applications is used the special
hysteresis control because the converter has resonant DC-link.
Resonant circuit is connected between the DC energy store and
the power switches of converter’s bridge (as shown in Fig. 2).
The disadvantages of classical hard switched converter (for
example the switches are subjected to a high-voltage stress,
and the switching power losses of a converter increases
linearly with the switching frequency) are eliminated or
minimized, because the power switches are turned “on” and
“off” when the voltage become zero. The current sensors (Lem
sensor, shown in Fig. 2) of PAPF monitor the line current in
real time and process the measured harmonics as digital
signals for control system with DSP.
The output of the power switches (IGBTs) through line
reactors (Lf, shown in Fig. 2) inject harmonic currents with an
exactly opposite phase to those that are to be filtered. The net
effect is an elimination of the harmonics and a clean sine wave
as seen by the feeding transformer.
Line Current
III. RESONANT DC-LINK AND DSP CONTROL UNIT
The resonant DC-link has implemented a limit voltage
circuit, so called the active clamp circuit. An active clamp
circuits as shown in Fig. 3 can limit the link voltage which is
shown in Fig. 5 (waveforms of resonant DC-link).
Voltage waveform of resonant DC-link on the Fig. 5 is
divided by 10 for better visualization. Without an active clamp
circuit are peaks of voltage resonant pulse slightly greater than
twice the DC input voltage.
The clamp factor k is related to the tank period Tk and
resonant frequency ϖ0 =
LC by

k (2 − k ) 
f0
.
= Tkϖ 0 = 2 cos −1 (1 − k ) +
(1)
fk
k −1 



That is for fixed value of k, Tk can be determined for a given
resonant circuit of the filter’s converter. For the value k = 1.5
is Tk = 7.65 LC .
clamp circuit
C1
To power
switches bridge
(IGBTs)
LR
Lem1
Switch
CR
Lem
is-ref
if
konst.
IGBT
Driver
System (DSP)
6
Fig. 3. Used circuit connections of resonant DC link inside the active filter
converter structure.
us-ref
Board of bidirectional voltage interface
Develop kit eZdsp
TMS320F2812
Lf
Shunt active power filter having converter with
resonant DC - link
Lr
Cr
Ed
Resonant
UTc
UTr
Capture
inputs
Digital
2x RS232
C2
Ed
Power
Switches
(IGBT)
C1
Energy
UZERO , ILr , I0
Store
Driver unit, Control circuits,
Measuring elements
Fig. 2. Block diagram of the power structure and connection with AC main.
C2
Iw, Iz
Control
Nonlinear
load
Us
serial reson. circuit
Control
to gates of power
Filter
Current
iL
Voltage
Load Current
iL
is
Lem2
Zero
DR
ifa,ifb,ifc
Xs
(k-1)Us
i0
ufa
us
DC
Store
Switch
DC
Analog
outputs
Analog
Fig. 4. Control unit based on DSP TMS320F2812.
PWM
outputs
ELECTRONICS, VOL. 14, NO. 1, JUNE 2010
95
Fig. 5. Voltage and current waveforms of resonant DC-link, output voltage
of the converter.
The control unit (which can be seen in the Fig. 4) was
designed and executed for the use of powerful microprocessor
system based on the Texas Instruments Digital Signal
Processor TMS320F2812. Generation of TMS320C28x™
digital signal controllers are the industry's first 32-bit
DSP-based controllers with on-board Flash memory and
performance up to 150 MIPS. Developed control unit was
design as universal control unit and can be used also for other
types of converters (in the lab was implemented and tested
with Three Level Inverter, Pulse Rectifier and so on). However
primary meaning of use should be for soft switched converter
which requires high-efficiency control system.
System action is divided to both fast algorithm for control of
resonant oscillation in resonant DC-link (as waveforms as is
shown at the bottom of Fig. 5) and compensating algorithm of
own power active filter and own utility routines.
Fig. 7. Current waveforms of simulated PAPF.
of line current. The second trace is the input current of device
with non-linear load (harmonics: I1 = 1 A; I3 = 0,2 A;
I6 = 0,1 A). The third trace is line current compensated by
proposed PAPF.
Next simulation waveforms of PAPF show Fig. 8. There are
three phase waveforms. The upper traces show an input current
of device with non-linear load and a line current (the
sinusoidal one) compensated by proposed PAPF in the phase
“U”. The second traces show an input current of device with
non-linear load and a line current (the sinusoidal one)
compensated by proposed PAPF in the phase “V”. The third
traces show an input current of device with non-linear load and
a line current in the phase “W”.
V. MEASURING RESULTS, REAL SAMPLE
IV. COMPUTER SIMULATION RESULTS
The following experimental results were acquired by PSpice
simulation model.
Fig. 6 shows the simulation waveforms of PAPF with soft
switched converter (having resonant DC-link). The upper trace
is current draw of the proposed PAPF (current is injected to
the power supply line by the filter). The second trace is line
current compensated by proposed PAPF. The third trace is the
input current of device with non-linear load (harmonics:
I1= 1 A; I3= 0,2 A; I9= 0,2 A).
Other simulation waveforms of PAPF having soft switched
converter shows Fig. 7. The upper trace is the required value
Fig. 6. Current waveforms of simulated PAPF.
Following pictures show practical realization results. In the
lab was designed and realized laboratory model of parallel
active power filter having resonant converter which shows
Fig. 9.
The testing was separated into 3 stages. In 1st stage was
tested the control unit (Fig. 4) and FFT (Fast Fourier
Fig. 8. Three phase simulation waveforms (phase “U”, phase “V”, phase
“W”) of PAPF with soft switched converter.
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ELECTRONICS, VOL. 14, NO. 1, JUNE 2010
Transformation) computing algorithms. In 2nd stage was tested
just resonant converter that was later implanted in PAPF
structure and in last stage was tested final behavior of
completed PAPF. On Fig. 9 you could see the measuring stand
with new laboratory sample of PAPF.
Fig. 10 shows 1st stage testing results. All waveforms are
inside signals of the control unit. The upper trace is input
current draw iLa of device with non-linear load (load consist of
resistor R = 20 Ω and inductor L = 60 mH). The second trace
is the computed 1st (fundamental) harmonic (required value) of
line current. The third trace is computed line angle ϕ1. The
waveforms are right computing results of FFT block (Fast
Fourier Transformation).
Fig. 11 and Fig. 12 shows 2nd stage testing results. All
waveforms are signals measured in the structure of applied soft
switched converter (new type of converter with resonant
DC-link as shows Fig. 3).
The upper trace of oscilloscope screen from Fig. 11 is
output current draw of converter with resonant DC-link which
was working as alone device (load consist of resistor R = 10 Ω
and inductor L = 100 mH). The second trace is output voltage
waveform of the converter and the third trace is detail
(ZOOM) of out voltage.
Fig. 11. Output waveforms of applied converter with resonant DC-link.
Fig. 12. Resonant DC-link waveforms, main part of practical realized soft
switched converter.
Fig. 9. Measuring stand with laboratory sample of PAPF.
Fig. 10. Calculated data – control signals of practical realized control unit.
Fig. 12 shows the waveforms confirming the right behavior
of practical realized resonant DC-link in case of activated
clamp circuit. The upper trace shows DC-link voltage
waveforms where we can see right behavior of clamp circuit.
Other trace shows waveform of DC-link differential current
IL – I0. Next two trace shows switching of “resonant” transistor
TR and clamp transistor TC. Below is mentioned detail
(ZOOM) of DC-link voltage and DC-link differential current
I L – I 0.
In our case (appliance of soft switched converter) was
necessary to evaluate performances of a new hysteresis current
control strategy for autonomous three phase parallel active
filter in harmonic currents elimination. Hysteresis control
strategy is based on currents errors and their derivatives
calculation each time the zero voltage vector is set at the AC
side of the inverter of the active filter.
Fig. 13 and Fig. 14 show last stage testing results. The first
trace in Fig. 13 is the input current of device with non-linear
load (non-linear load: 3. phase non-controlled rectifier loaded
by resistor R = 20 Ω and inductor L = 60 mH). The second
trace is output current draw of the proposed PAPF (current is
injected to the power supply line by PAPF). The second trace
is line current compensated by proposed PAPF.
Fig. 14 shows current waveforms in case of load change
(from 0% to 100%). There you can see the right and quick
adaptation. The first draw in this figure is the input current of
device with non-linear load (non-linear load: 3. phase non-
ELECTRONICS, VOL. 14, NO. 1, JUNE 2010
Fig. 13. Measured waveforms show right behavior of realized PAPF with
soft switched converter.
97
type of parallel active power filter. The operation principles of
parallel active power filter with resonant converter have been
presented and analyzed in this paper.
The simulation results proved the viability of using resonant
circuits (resonant DC-link) for implementation into the
structure of parallel active power filters. The simulation
circuits were designed as a simplified model. However, for
calculation of simulative diagram is needed high – powered
computer.
Practical realization was based on simulation results, which
are presented in this paper. Presented results confirm right
behavior of developed PAPF working as soft switching device
(in mode of Zero Voltage Switching). This conception brings
the advantages of resonant converters. Such as savings of
electrical energy (power switches are turned-on and turned-off
when the voltage becomes zero) and much better
electromagnetic interference properties.
ACKNOWLEDGMENT
The project is supported by Grant Agency of Czech
Republic (GAČR), project No. 102/09/P665, for which author
express their sincere gratitude.
REFERENCES
[1]
[2]
[3]
Fig. 14. Measured waveforms show right behavior of realized PAPF with
soft switched converter.
controlled rectifier loaded by resistor R = 20 Ω and inductor
L = 60 mH).
The second trace is output current draw of the proposed
PAPF (current is injected to the power supply line by PAPF)
and last trace is line current compensated by proposed PAPF.
[4]
[5]
[6]
[7]
VI. CONCLUSION
Active filters have wide application for controlling
harmonic currents from nonlinear loads. The device and
techniques described in the paper show the possible way to
correct current harmonics at power supply system by the new
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